messages: Add support for right-to-left messages.

This implements right-to-left message automatic detection support in
the compose box as well as the message feed.  Full unit tests and
support in the message-editing UI are for future work (as are
potentially more fancy things like supporting things like
right-to-left multi-word names for users/streams/etc.).

Fixes #3123.

static/js/message_list_view.js

@@ -406,6 +406,12 @@ MessageListView.prototype = {
                 blueslip.warn('Only DOM nodes can be passed to _post_process_messages');
             }
             var row = $(dom_message);
+            var content = row.find('.message_content');
+
+            // Set the rtl class if the text has an rtl direction
+            if (rtl.get_direction(content.text()) === 'rtl') {
+                content.addClass('rtl');
+            }
 
             // Save DOM elements by id into self._rows for O(1) lookup
             if (row.hasClass('message_row')) {